1. Field of the Invention
Many people, particularly the aged, develop cataracts necessitating the removal of the natural crystalline lens from within their eye. Intraocular lenses (IOLs) are artificial (prosthetic) lenses that are used as a replacement for the natural lens of the human eye. The present invention relates generally to multifocal diffractive lenses, and more particularly to trifocal diffractive IOLs. Trifocal diffractive IOLs have three distinct powers that provide a patient, who has undergone cataract surgery, with far, intermediate, and near vision prescriptions.
2. Description of the Prior Art
A diffractive lens generally comprises a number of concentric annular zones of equal area with optical steps separating the adjacent annular zones. The width of the zones determines the separation between the diffractive powers (vision prescriptions) of the lens, the separation of the diffractive powers increasing with decreasing zone width.
a. Diffraction Bifocals
Diffractive lenses have been known and analyzed since the 1800s, but it wasn't until the 1960s that they were first used to correct human vision. The earliest such ophthalmic diffractive lens had a Fresnel Phase Plate design with “flat” zones, as disclosed in 1967 by Gunter Überschaar in his German Patent Publication 1,235,028. This Fresnel Phase Plate design used ½ wave deep flat steps which produced a bifocal lens that divided light equally between the +1 and −1 diffraction orders.
We can better understand the Überschaar lens by referring to FIG. 1a where we see a front view of a contact lens showing the boundaries r1, r2, r3, . . . of the flat annular zones 1 according to his invention. FIG. 1b is a cross-section view showing the flat annular zones 1 comprising the anterior diffractive surface 2 of the contact lens 3 of FIG. 1a. The flat annular zones 1 are shown schematically in FIG. 1c with the vertical axis representing step height S and the horizontal axis representing r2, where r is the radial distance from the center of the contact lens. Since all the annular zones in this diffractive lens have equal area, the zone boundaries r1, r2, r3, . . . are equally spaced in r2-space.
In the 1980s Cohen (U.S. Pat. Nos. 4,210,391; 4,338,005; 4,340,283; 5,054,905; 5,056,908; 5,117,306; 5,120,120; 5,121,979; 5,121,980; 5,144,483) and Freeman (U.S. Pat. Nos. 4,637,697; 4,642,112; 4,655,565; 4,641,934) disclosed the first ophthalmic diffractive lenses with blazed (angled) zones exhibiting a saw-toothed profile. These blazed designs used ½ wave deep substantially parabolic profiles which created bifocal lenses that divided light equally between the 0th and +1 diffraction orders.
FIG. 2a is a cross-section of the blazed annular zones 4 comprising the anterior diffractive surface 5 of an IOL 6 according to the inventions of Cohen and Freeman. The blazed annular zones 4, of this IOL, are shown schematically in FIG. 2b with the vertical axis representing step height S and the horizontal axis representing r2, where r is the radial distance from the center of the contact lens.
We also see in FIG. 2b that each blazed annular zone 4 comprises a discrete step surface 4a and a blazed surface 4b. In r2-space the blazed surfaces 4b take a linear form, and the zone boundaries r1, r2, r3, . . . are equally spaced. But, it should be recognized that, when graphed in r-space (as opposed to r2-space), not only will the zones be unequally spaced, but the profile of the blazed surfaces 4b will take on a quadratic form as illustrated in FIG. 2c. 
In 2003, Fiala (U.S. Pat. No. 6,536,899) disclosed a bifocal that divided each annular zone into two sub-zones. The sub-zones were designed as a way to eliminate the sharp steps of the standard blazed step bifocals. In his lens design, each annular zone comprised a main sub-zone of larger area and a phase sub-zone of smaller area and narrower width. In particular, he designed his phase sub-zones to cause the necessary phase shift needed to replace the optical steps of a diffractive lens.
The annular zones of the Fiala invention, are shown schematically in FIG. 3 with the vertical axis representing the step height S and the horizontal axis representing r2, where r is the radial distance from the center of the lens. The annular zones of this lens, bounded on the outside by the radii r1, r2, r3, . . . , are each divided into two sub-zones 7a and 7b which are generally of unequal area. While the sub-zones are of unequal area, the ratio of their areas remains unchanged from zone to zone.
b. Diffraction Trifocals
By the early 2000s, commercial diffractive IOLs were widely available as bifocals with blazed zone designs providing a split of light between the 0th and +1 diffraction orders. However, in 1994, Swanson (U.S. Pat. No. 5,344,447) had already disclosed a trifocal diffractive IOL based on a Fresnel Phase Plate design. This design uses ⅓ wave deep flat steps, thereby producing a trifocal lens that divides light equally between the +1, 0th, and −1 diffraction orders. A modification of this trifocal design was disclosed in 1998 by Kosoburd (U.S. Pat. No. 5,760,871).
Then, in 2011, Schwiegerling (U.S. Pat. App. Pub. 2011/0292335) and Gatinel (Intn'l. Pat. App. Pub. WO 2011/092169) proposed designs r a diffractive trifocal with blazed steps. These designs use equal area blazed annular zones with alternating step heights, thereby creating a trifocal lens that divides light between the 0th, +1, and +2 diffraction orders.
The alternating annular zones 8a and 8b of these trifocal designs, are shown schematically in FIG. 4. We can see that all of the zones 8a have the step height Δa, while all of the alternate zones 8b have the step height Δb, where the step height Δb is not equal to the step height Δa.
c. Apodization
Meanwhile, it was seen to be advantageous to find designs that would provide a varying split of light between the different diffraction orders as the pupil of the eye opens and closes. In 1997 Simpson (U.S. Pat. No. 5,699,142) disclosed an “apodized” design wherein the light distribution between the 0th and +1 diffraction orders could be controlled by varying the step height (zone depth) of the individual zones. Since then, within the context of ophthalmic diffractive lenses, apodization has come to mean the gradual reduction of the step heights starting at the center of the lens and moving outward toward the periphery. Apodization has been widely adopted for multifocal diffractive IOLs and allows the light distribution between the 0th and +1 diffraction orders to vary as the pupil of the eye opens and closes. FIG. 5a illustrates the design profile of a typical apodized diffractive bifocal wherein the peaks of the blazed steps 9 follow a line of apodization 10.
In addition to the constant depth reduction across each zone progressing from the central zone outward, Simpson included a phase shift across each zone, again progressing from the central zone outward. This phase shift was introduced to allow the light from the various zones of different depths to all remain in phase. FIG. 5b illustrates the design profile of a typical apodized and phase shifted diffractive bifocal wherein the peaks of the blazed steps 11 follow a line of apodization 12 and the blazed steps are also phase shifted to center on a phase shift line 13.
Schwiegerling and Gatinel also incorporated apodization schemes in their trifocal inventions. FIG. 6a illustrates their trifocal design with apodization of the alternate blazed steps 14a and 14b. FIG. 6b illustrates their trifocal design with both apodization and phase shifting of the alternate blazed steps 15a and 15b. Finally, FIG. 6c illustrates a double apodization scheme for a diffractive trifocal lens as proposed by Schwiegerling wherein the peaks of the alternate blazed steps 16a and 16b each follow the different, non-parallel, lines of apodization 17 and 18 respectively.